1. Field of the Invention
The present invention relates to a thermal recording device and method for thermal printers, and particularly to an area gradation thermal recording device and method for providing a half-tone of image by changing the recording area of each dot within one pixel. More particularly, the present invention relates to a method and device for compensating the influence of heat energy accumulated in a thermal recording head onto the recording density.
2. Description of the Related Art
Conventionally, there are two types of thermal recording: one type is a direct thermal recording method wherein recording paper is directly heated to record dots on the recording paper, and the other type is a thermal transfer method wherein an ink is transferred from an ink film onto recording paper. The thermal transfer method includes wax transfer type recording wherein melted or softened ink is transferred to the recording paper, and sublimation type recording wherein a dye is sublimated or diffused from the ink film to the recording paper.
In order to provide a half-tone image, an area gradation method has been proposed. According to this method, one pixel is divided into a plurality of sub-lines. Each time a thermal element faces to one sub-line to be recorded, a drive pulse is supplied to the thermal element to be energized to record a dot, wherein the gradation of a pixel, that is, the size of the dot changes according to the number of sub-lines on which the dot is recorded. This area gradation method is effective for a printer which cannot precisely control the density of a dot itself, such as a thermal wax transfer type printer which heats the back of an ink film and transfers melted or softened ink onto a recording sheet.
Printers incorporating such an area gradation method may use a recording head (thermal head) having a plurality of thermal elements arranged in an array. In order to prevent trailing of ink that may be caused by heat energy accumulated in the recording head, various devices have been provided for radiating the accumulated heat energy from the recording head, so that the heated thermal elements are rapidly cooled.
However, if the recording head is very cool prior to the start of recording, it is impossible to record ink at sufficient density on the first sub-line of a pixel by the same drive pulse as used for recording the following sub-lines. In view of this, Fuji Photo Film Co., Ltd. has suggested a method wherein a booster pulse having a large width is applied for rapidly heating up the thermal element to a temperature above which recording is possible, when recording the first sub-line of each pixel (Japanese Patent Application No. 2-15886 corresponding to Japanese Laid-open Patent Application No. 3-219969). However, this method has a problem in that a so-called shading effect may be caused by heat energy accumulated in the recording head when recording dots continuously at maximum density by continuously driving the same thermal element. Because of the shading, the density of the dots undesirably changes as the recording proceeds and the density of the latter dots exceeds a desirable value.